80 store credit for an xbox is not bad at all. There are so many Vice City games in circulation that expecting more than 6.50 is expecting too much. It's worth trading in your games sometimes, like when you want to get a new game or a brand new console and you don't have the cash. Or when the game you bought either sucks or you don't dig it. Also out of all the places, Gamespot is the best retail place there is. The other places like Blockbuster, Hollywood Video, Mediaplay, Toys r Us, Electronics Boutique are much worse. You could use Ebay, but thats a personal taste as to whether you like dealing online, waiting, shipping through mail etc.
Also if you want the best cash for your games you gotta sell the right ones at the right time, it can make a big difference. Keeping track of a games circulation, current popularity, and price new are just a few parts of the equation.